The so-called sleep of leaves is so conspicuous a phenomenon that it was
observed as early as the time of Pliny;* and since Linnaeus published his
famous Essay, 'Somnus Plantarum,' it has been the subject of several
memoirs. Many flowers close at night, and these are likewise said to sleep;
but we are not here concerned with their movements, for although effected
by the same mechanism as in the case of young leaves, namely, unequal
growth on the opposite sides (as first proved by Pfeffer), yet they differ
essentially in being excited chiefly by changes of temperature instead of
light; and in being effected, as far as we can judge, for a different
purpose. Hardly any one supposes that there is any real analogy
* Pfeffer has given a clear and interesting sketch of the history of this
subject in his 'Die Periodischen Bewegungen der Blattorgane,' 1875, P. 163.
[page 281]
between the sleep of animals and that of plants,* whether of leaves or
flowers. It seems therefore, advisable to give a distinct name to the
so-called sleep-movements of plants. These have also generally been
confounded, under the term "periodic," with the slight daily rise and fall
of leaves, as described in the fourth chapter; and this makes it all the
more desirable to give some distinct name to sleep-movements.
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